[migractions] add a dropped_constraints() migration command
Example usage:
>>> with dropped_constraints('MyType', 'myattr',
... UniqueConstraint, droprequired=True):
... add_attribute('MyType', 'myattr')
... # + instructions to fill MyType.myattr column
...
>>>
# copyright %(year)s %(author)s, all rights reserved.
# contact %(author-web-site)s -- mailto:%(author-email)s
#
%(long-license)s
"""%(distname)s schema"""